Transformation constraint does not contain mapping?
Check your axis.
Recently answered this question re bone drivers.

In the armature (data) properties viewport display tab, turn on Axes. Or use translate tool and put in LOCAL transform orientation mode. Your screenshot is in GLOBAL.  This will show the directions of the local axes. For bones Y is head to tail.

Bone on right is root bone, when moved locally in X, rotates other bone in Z.

Mapped from Local X location (0, 1) to Local Z rotation (0, 360), with Z source targeting X
Your axis alignment may differ, can only tell Y from screenshot. Change to suit, but have feeling your wheels will spin if you move car sideways 8).
Note, if you know the diameter of your wheel (d),  move pi x d for every rev. to make rotation look real.  Generally for wheels would spin a bone on its Y axis, but it's arbitrary.
PS. Whoops, May have got my mapping spinning wrong way, (negate a value in to or from) but you get the gist.)